Fabrication of Nanoflowers and Other Exotic Patterns

Hardev Singh Virk Professor Emeritus, Eternal University, Baru Sahib, HP, India

hardevsingh.virk@gmail.com

Keywords: Electro-deposition, Hydrothermal method, Anodic Alumina & Polymer templates, Nanoflowers, Exotic patterns, Nucleation rate.

Abstract. A wide variety of metallic and metal oxide nanoflowers and other exotic patterns have been fabricated using different techniques. We have created copper and cupric oxide nanoflowers using two different techniques: electro-deposition of copper in polymer and anodic alumina templates, and cytyltrimethal ammonium bromide (CTAB)-assisted hydrothermal method, respectively. Zinc oxide and manganese oxide nanoflowers have been synthesized by thermal treatment. Characterization of nanoflowers is done in the same way as for nanowires using XRD, SEM, TEM and FESEM. Scanning Electron Microscope (SEM) images record some interesting morphologies of metallic copper nanoflowers. Field Emission Scanning Electron Microscope (FESEM) has been used to determine morphology and composition of copper oxide nanoflowers. X-ray diffraction (XRD) pattern reveals the monoclinic phase of CuO in the crystallographic structure of copper oxide nanoflowers. Nanoflowers find interesting applications in industry. There is an element of random artistic design of nature, rather than science, in exotic patterns of nanoflowers fabricated in our laboratory.

1. Introduction

During the last decade, exhaustive reviews [1-4] have been published on metal nanostructures. A series of various nanoflowers and nanoflower-like structures have been obtained, depending on reaction conditions, such as reagents ratio, temperature and other conditions. Nanoflower structure may consist of such more simple nanostructures; as nanorods, nanowalls, or nanowires. Current and possible applications of nanoflowers as optoelectronics devices or sensors, in catalysis, and solar cells caused a definite interest in them. Nanoflowers of almost all metals have been reported in the form of elemental nanoflowers; metal oxide nanoflowers; nanoflowers of hydroxides and oxosalts; sulphide, selenide and telluride nanoflowers; nitride and phosphide nanoflowers; nanoflowers formed by organic and coordination compounds [1].

Flower-like cupric oxide (CuO) nanostructures have been prepared via cytyltrimethal ammonium bromide (CTAB)-assisted hydrothermal method [5]. CTAB is a useful surfactant that has been widely used in fabricating the nanomaterials to control the morphology. Cao et al. [6] reported CTAB-assisted hydrothermal synthesis of CuO of various morphologies such as rod-like, spheriodal, hexahedron and other irregular structures. Cupric oxide (CuO) has potential applications in many fields, such as superconductor [7], gas sensor [8], catalyst [9], magnetic storage media [10] and lithium battery [11]. Synthesis of CuO nanoflowers and their application as an H2O2 sensor has been reported by Gu et al. [12]. They successfully synthesized CuO nanoflowers by heating Cu(NO3)2⋅3H2O and NH3·H2O for 6 h at low temperature. No additives, surfactants and templates were used in their synthesis.

Hybrid organic–inorganic nanoflowers were discovered by accident by Ge et al. [13] when they added 0.8 mM CuSO4 to phosphate buffered saline (PBS) containing 0.1 mg/ml bovine serum albumin (BSA) at pH 7.4 and 25 0C. After three days, a precipitate appeared with porous, flower-like structures. Hybrid organic–inorganic nanoflowers using copper (II) ions as the inorganic component and various proteins as the organic component are being investigated for promising biosensor applications. The protein molecules form complexes with the copper ions, and these complexes become nucleation sites for primary crystals of copper phosphate. Interaction between

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the protein and copper ions then leads to the growth of micrometre-sized particles that have nanoscale features and that are shaped like flower petals. When an enzyme is used as the protein component of the hybrid nanoflower, it exhibits enhanced enzymatic activity and stability compared with the free enzyme. This is attributed to the high surface area and confinement of the enzymes in the nanoflowers.

Copper phthalocyanine nanoparticles and nanoflowers have been investigated by Karan et al. [14]. Metal phthalocyanines (MPcs) are very well known dye pigments having similarity in structure with biological molecules chlorophyll and haemoglobin. These materials have shown many interesting properties and high vacuum evaporation has become the most widely used technique for the deposition of MPc films [15–18]. Attempts have been made to use such films as molecular components in a number of electronic and optoelectronic devices [19-21]. For the better technological applications of phthalocyanines as organic semiconductors, electronically active organic molecules and components for molecular electronics it is necessary to study, under various experimental conditions, the interactions of these molecules with various electrode materials like gold, silver etc. As among the metal substituted phthalocyanines, copper (II) phthalocyanine (CuPc) has been found to have superior properties [22–24].

Author’s group [25-30] has reported fabrication and characterization of copper nanowires and some exotic patterns of polycrystalline copper recently using electrodeposition technique of template synthesis. (CTAB)-assisted hydrothermal method [5] has also been exploited for synthesis of CuO nanoflowers. A comprehensive investigation is planned to exploit the industrial applications of copper nanoflowers, for example, field emission properties.

2. Experimental Investigations

There are several different methods for fabrication of nanoflowers as reported in various reviews [1-4]. We followed two different routes for preparation of copper nanoflowers: electro-deposition technique using template synthesis and CTAB-assisted hydrothermal method.

Electro-deposition technique used in our experiments [25-30] is similar in principle to that used for the electroplating process and has been vividly described in the Chapter on Nanowires. Commercially available polycarbonate membranes (Sterlitech, USA) of 25 mm diameter with pore density of 108 pores/ cm2 and pore diameter of 100 nm were selected for this experiment. The main purpose of this experiment was to fabricate nanowires of copper. However, to our utter surprise, we failed in our mission and what we got was a blessing in disguise. We observed formation of some exotic patterns, including synthesis of nanoflowers.

A second set of experiments was completed using commercial anodic alumina membranes (AAM) (anodisc 25 made by Whatman) having an average pore diameter of 200 nm, a nominal thickness of 60 µm and a pore density of 109 pores/cm2, as templates.

The electrochemical cell, fabricated in our laboratory using Perspex sheets, was washed in double distilled water. A copper rod of 0.5 cm diameter was used as a sacrificial electrode (anode). The cathode consists of copper foil attached to polymer template by an adhesive tape of good conductivity. The electrolyte used had a composition of 20 gm/100ml CuSO4.5H2O + 25% of dilute H2SO4 at room temperature. The inter-electrode distance was kept 0.7 cm and a current of 2mA was applied for 10 minutes. After electrodeposition, the polymer template was dissolved in dichloromethane to liberate copper nanoflowers from the host matrix. Copper nanowires were produced generally using AAM template but in some cases, nanoflowers appeared on the periphery of template due to poor contact. The Scanning Electron Microscope (JEOL, JSM 6100) was used to record top and side views of grown nanostructures at an accelerating voltage of 20kV using different magnifications.

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For hydrothermal synthesis [5], analytical grade (Loba Chemicals) copper chloride dihydrate, CuCl2. 2H2O, and sodium hydroxide, NaOH, were used as precursors and CTAB as surfactant. All the chemicals were directly used without further purification and de-ionized water was used for preparation of solution.

In a typical synthesis, the copper chloride solution was prepared by dissolving 0.85g (5mmol) of CuCl2.2H2O in 20 mL de-ionized water. Subsequently, the copper chloride solution was slowly dropped into 50mL of NaOH solution (3mol.L-1) under vigorous stirring. The blue-colored precursor was obtained. 1g of CTAB (3 mmol) was added to the blue-colored precursor and stirred vigorously for 30 min at 50 0C to ensure the complete dissolution of CTAB. This reaction solution was then transferred to a 50 mL Teflon-lined stainless steel autoclave and heated at 150 0C for 12 h in an electric oven. After reaction, autoclave was allowed to cool to room temperature. The resulting black precipitate was centrifuged and washed thoroughly with de-ionized water and ethanol. Then the precipitate was dried in drying oven at 50 0C for 24 h. Finally, the reaction products were calcined in a furnace at 500 0C for 2h in an ambient air atmosphere.

Among metal oxides, ZnO has been most investigated and its morphologies usually include nanowires, nanorods, nanobelts, nanorings, nanosheets, nanodisks, nanoflowers, nanoneedles, nanonails, nanopencils, and nanoflakes [31-33]. As another important application of ZnO nanoflowers, a new amperometric biosensor for hydrogen peroxide was developed [34]. Rapid response, expanded linear response range, and excellent stability were found for this biosensor. Flower-shaped ZnO nanostructures composed of ZnO nanosticks have been synthesized by the solution process followed by thermal treatment by Wang et al. [35].

Synthesis of ZnO nanoflowers was carried out by solution process at 180°C using zinc acetate dihydrate, sodium hydroxide and PEG-20000 as source materials. For the synthesis, equivalent volume zinc acetate dihydrate (0⋅5M) and sodium hydroxide (5M) were mixed to obtain solution A. 1g PEG-20000 was dissolved in 4 ml of water by sonication to obtain solution B. The solution B was then added into 5 ml solution A to obtain solution C. 55 ml of 1-octanol was added to solution C under stirring at room temperature to obtain solution D. Then solution D was transferred to Teflon-lined stainless steel autoclave which was then heated at 180°C for 4 h in an electric oven. The ZnO white powder could be obtained after filtering, washing and drying. The mechanism of flower-shaped ZnO formation is illustrated (Fig. 1). SEM images of ZnO nanoflowers are also shown (Fig. 2) [35]. The X-ray diffraction pattern of synthesized ZnO powder confirmed that the ZnO nanoflowers conform to wurtzite hexagonal structures. More detailed information about ZnO nanoflowers is reported elsewhere [36-40]. SEM image of ZnO teterapods is shown in figure 3 [41].

Fig. 1 Illustration of the formation mechanism of flower-shaped ZnO [35]

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Fig. 2 SEM images of ZnO nanoflowers: (a, b) represents overall morphology, and (c, d) the detailed magnified view of an individual flower [35]

Fig. 3 SEM image of ZnO tetrapods (snowflakes) [41]

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Synthesis of MgO nano-structures production by chemical vapor transport and condensation, with its distinct nanoforms could be obtained [42]. Single crystalline MgO nanoflowers, consisting of MgO nanofibers (20-80 nm), were synthesized via conventional evaporation method using the high-purity magnesium powders and distilled water as starting materials [43]. The obtained MgO nanoflowers have a much higher relative dielectric constant as compared with MgO micropowders and may be useful in providing insight into the formation of microbiological systems and reinforcing composite materials. The different steps of formation mechanism of MgO nanoflowers have been discussed by Fang et al. [44] and it includes Mg particles formation on the Si substrate, formation of MgO clusters as nucleation centers on the magnesium melt surface and the nucleation of short MgO nanofibers, and then the growth of the MgO nanofibers into MgO nanoflowers (Fig. 4).

Elegant three-dimensional MoS2 nanoflowers (Fig. 5), consisting of tens to hundreds of hexagonal petals (100-300 nm wide and several nm thick) and exhibiting excellent field emitter properties, were uniformly formed via heating a MoO2 thin film formed on a Mo foil in a sulphur vapor atmosphere at 950-1000 oC [45]. Hydrothermal synthesis leads to different MoS2 morphologies, in particular nanoflowers at much longer aging period [46].

Fig. 4 SEM Image of Mg (OH)2 nanoroses [41]

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Fig. 5 SEM images of MoS2 flower-like nanostructures: (a) low magnification view; (b, c) high magnification view [45] Crystalline GaP nanoflowers (Fig. 6) with cubic structure composed of numerous GaP nanowires were synthesized through heating InP and Ga2O3 powders [47]. The authors proposed that GaP nanoflowers may be valuable for future nanodevice design. Similar nanoflower-like GaP nano-structures, constituted also by numerous nanowires (diameters of 80-300 nm; lengths varying from several to tens of µms), were fabricated by another route: through the close-spaced vapor transport technique (CSVT) by growing on crystalline GaAs, using a GaP powder source in the absence of any catalyst [48]. For the synthesis of the protein–inorganic hybrid nanoflowers, 20ml of aqueous CuSO4 solution (120 mM) in molecular-biology-grade water was added to 3 ml of PBS (pH 7.4) containing proteins with different concentrations, followed by incubation at 258 0C for three days. For SEM analysis, the suspension of the prepared nanoflower was filtered and dried on a membrane (pore size, 0.1mm) and sputter-coated with gold. For TEM analysis, a drop of the suspension of the prepared nanoflower was added to a carbon grid and dried at room temperature. Figure 7 shows the SEM images of hybrid nanoflowers at different protein concentrations.

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Fig. 6 SEM image of GaP nanoflowers [47]

Fig. 7 SEM images of hybrid nanoflowers. Column 1 (from left to right): α-lactalbumin, column 2: laccase, column 3: carbonic anhydrase, and column 4: lipase, at protein concentrations of 0.5 mg/mL (a-d), 0.1 mg/mL (e-h), and 0.02 mg/mL (i-l). From this figure, the morphological changes with concentration conditions are clearly evident [adopted from ref. 13].

Karan et al. [14] synthesized copper phthalocyanine (CuPc) nanoflowers in the following way: Gold

single crystal with (111) surface orientation was used as a target. Clean gold film surfaces were

prepared on the surface of clean quartz plates by D.C. sputtering (7 KV, 10 mA) for 4 min followed

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by annealing for 1 h at 500 oC and 750 oC under vacuum. The substrates annealed at 500 oC and 750 oC are referred to as Au-500 and Au-750, respectively. The CuPc source contained in a

molybdenum boat was resistively heated in the high vacuum chamber. The CuPc powder (β form,

dye content about 97%, obtained from Aldrich, USA) was used after repeated degassing of the

source prior to deposition. Deposition was noted to occur when the chamber pressure could no

longer reduce to the base pressure and the current through the boat was kept constant at 20 A. CuPc

films were deposited at a chamber pressure of 10 Torr. The thickness of the deposited CuPc film

and the rate of deposition were maintained at 100 nm and 0.6 A˚/s, respectively using a quartz

crystal microbalance (Hitech, model DTM-101). The temperature of the substrates during

deposition was kept at room temperature (30 oC).

FESEM images of CuPc thin films deposited at room substrate temperature are shown in Fig. 8.

Fig. 8(d) shows one nanoflower of diameter around 850 nm. The flower consists of some

nanoribbons of CuPc of diameter nearly 25 nm and few nanometers in length. The structures of

such nanoflowers arise due to the interaction of CuPc molecules with annealed gold template (Au-

750), which was the same type of surface having spherical and elliptical particles of various sizes.

The initial nucleation of CuPc molecules occurs at room substrate temperature on the gold particles

and form flower like structure through the process of self-organization.

Fig. 8 FESEM images of (a) CuPc nanoparticles deposited on quartz, (b) dense and aggregated CuPc nanoparticles deposited on Au-500, (c) arrangement of spherical and elliptical CuPc nanoflowers of different sizes deposited on Au-750 and (d) CuPc nanoflower at higher magnification [adopted from ref. 14].

Chybczyńskawe et al. [49] report for the first time on the microwave assisted hydrothermal synthesis of bismuth ferrite BiFeO3 (BFO) nanoflowers. Bismuth ferrite, even in bulk is an unusual material because it belongs to magnetoelectric (ME) multiferroics that exhibit simultaneously

166 Functional Nanomaterials and their Applications

charge and magnetic ordering with some mutual coupling between them at room temperature [50-52]. These materials have recently attracted world-wide attention because of their interesting physical properties and large technological potential to be applied in spintronic devices, and in piezoelectric devices as THz radiation emitters [53].

BFO nanoflowers were synthesized by means of microwave assisted hydrothermal method using bismuth and iron nitrates as precursors: Bi(NO3)3·5H2O, Fe(NO3)3·9H2O. The nitrates together with Na2CO3 were added into a KOH solution of a molar concentration of 6 M. The mixture was next transferred into a Teflon reactor (XP1500, CEM Corp.) and loaded into a microwave oven (MARS 5, CEM Corp.). The reaction was carried out at the same temperature for all samples (200 0C) for a short time equal to 20 min (samples labeled ST) or for one hour which was the long time synthesis (samples labeled LT). After processing, BFO powders were first cooled to 20 0C, next collected by filtration kit, rinsed with distilled water and placed in a dryer for 2 h. The final products were brown powders of BFO agglomerates. In this way a set of ST and LT samples composed of grains in the shape of nanoflowers (Fig. 9) was synthesized [49].

Fig. 9 SEM micrograph of bismuth ferrite (BFO) nanoflowers [adopted from ref. 49]

3. Results and Discussion

There is as yet no specific theory to explain exotic patterns developed during electro-deposition of copper in anodic alumina or polymer templates. A speculative explanation [54] is provided on the basis of over-deposition. During the growth of copper nanowires in the template pores, the current remains nearly stable until the wires arrive at the template surface. If the electro-deposition process is not stopped at this stage, the current keeps on rising very gradually leading to over-deposition of copper. The exotic patterns in the form of micro-flowers having their petals in nanometer dimension, copper buds leading to mushroom effect and double pyramid-shaped copper crystals have been observed in our investigations [25-30]. It has also been reported by National Institute of Technology (NIT), Kurukshetra [55] that over-deposition of copper in polymer templates may lead to formation of metallic micro-rose having petals in nanometer dimensions [Fig. 10]. Flower- like morphology of metal over-deposits has been attributed to the changes in hydrodynamic conditions due to excessive hydrogen evolution during electro-deposition process [55]. This explanation seems to be a tentative one which fails to justify the morphology and geometrical pattern of the flower shape formed.

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Fig. 10 Copper metallic Rose flower grown in polymer template [adopted from ref. 55]

During our recent experiments, we observed that the growth of nanoflowers depends upon two factors: cathode over- potential and conductivity of the cathode surface. If the conducting film is used for the cathode surface, Cu ions will tend to deposit into nanochannels of polymer template, otherwise they tend to grow laterally on the cathode surface. The deposition of copper takes place only when the potential of the cathode is lower than the equilibrium electrode potential of the electrolytic cell; hence, a certain magnitude of cathode over-potential is necessary. The relationship between the over-potentials and the nucleation rates was given by Erdey-Gruz and Volmer [56] as follows:

N = a e-b/ηk2 (1)

where N is the nucleation rate; ηk is the cathode over-potential; a and b are constants. It can be seen that the higher the over-potential, the higher the nucleation rates of growth. At a certain optimum over-potential, nanoflower fabrication occurs.

SEM micrographs of copper nanoflowers fabricated using polymer templates are shown in Figs. 11-13. Figure 11 shows the deposition of copper in the form of flower petals. These flower petals, also called nanoflakes [57], are believed to have resulted from a nucleation process governed by an aggregation-mediated crystallization. Figure 12 depicts two different shapes of copper nanoflowers. Surprisingly, the morphology of flowers in Fig. 12 (a) is almost identical in shape and pattern but in Fig. 12 (b), it almost resembles with marigold flowers in a garden. Figure 13 represents lilly-like botanical plants of copper metal grown on the cathode surface. The beauty of these experiments is that no identical patterns are produced on repeating the experiment. It remains an enigma and defies scientific explanation.

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Fig.11 SEM micrograph of copper nanoflower petals grown in polymer template

(a) (b)

Fig. 12 (a, b) SEM micrographs of copper nanoflowers grown in polymer template

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(a) (b)

Fig. 13 (a, b) SEM micrographs of lilly-like copper plants grown in polymer template

Yang et al. [58] have found a beautiful analogy between nanoflower arrays of rutile TiO2 and garden-cosmos (Fig. 14), as reported by Liang et al. [59] in a recent publication. The formation mechanism of rutile TiO2 nanoflower arrays has been discussed in detail [59].

(a) (b)

Fig. 14 (a) SEM image of rutile TiO2 nanoflowers, and (b) a photograph of a garden-cosmos [58,59]

In case of anodic alumina template, copper nanowires [25] were grown in central region and nanoflowers in the peripheral zone (Fig. 15). It clearly proves the hypothesis of differential deposition of copper on the cathode surface. It is difficult to determine the exact conditions under which nanoflowers are synthesized along with nanowires. Our investigations reveal that chance plays a predominant role in growth of nanoflowers. The most disturbing feature of our study is that different types of nanostructures are created under similar experimental conditions. There is an element of random artistic design in nanoflowers fabricated in our laboratory. However, there is one satisfaction that all these exotic patterns find some analogue in nature.

170 Functional Nanomaterials and their Applications

(a) (b)

Fig. 15 (a, b) SEM micrographs of copper nanoflowers grown in anodic alumina template

It has been observed that in addition to nanoflowers, some other exotic patterns are also created during electrodeposition in anodic alumina template. An interesting feature is mushroom-like growth of copper buds of different shapes and sizes (Fig. 16). We call it the ‘Mushroom effect’. There is no specific reason for the growth of copper buds except the over-deposition of metallic copper on the cathode surface.

(a) (b)

Fig. 16 (a, b) SEM micrographs of copper buds grown in alumina template (Mushroom Effect)

FESEM micrograph (Fig. 17) shows CTAB-assisted hydrothermal synthesis of CuO nanoflowers. The explanation for CTAB-assisted hydrothermal method of CuO flowers fabrication is given by Zou et al. [5]. According to the literature survey, the formation mechanisms of the flower-like CuO nanostructures were different when different preparation methods were used. Yu et al. [57] prepared the flower-like CuO nanostructures by reaction between a Cu plate and a KOH solution at room temperature and demonstrated their field emission properties. They speculated that the nanoflower was a representative morphology of spherulite formed by radiating growth from a center or a number of centers and the [Cu (OH)4]

2- complexes played a key role in the growth of nanoflowers.

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Zhu et al. [60] used a facile chemical procedure capable of aligning CuO nanoparticles on graphene oxide and observed that the nanocomposite exhibits a high catalytic activity for the thermal decomposition of ammonium perchlorate. Teng et al. [61] synthesized the flower-like CuO nanostructures by hydrothermal process using copper threads as precursor. They investigated the influences of hydrothermal temperature and hydrothermal time on the nanostructures and reported the use of CuO as a sensor for catalysis. The formation of the flower-like structure was controlled not only by the growth thermodynamics, but also by the growth kinetics.

(a) (b)

Fig. 17 (a, b) FESEM micrographs of CuO nanoflowers prepared by hydrothermal method

Fig. 18 XRD spectrum of monoclinic copper oxide nanoflowers

Position [°2Theta] (Copper (Cu))

10 20 30 40 50 60 70

Counts

0

2000

4000

CuO

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Table 1 XRD spectrum peaks data of CuO nanoflowers

Pos. [°2Th.]

FWHM [°2Th.]

d-spacing [Å]

Rel. Int. [%]

Area [cts*°2Th.]

32.7906 0.1338 2.73127 8.01 55.93

35.8066 0.2509 2.50784 100.00 1309.41

38.9983 0.1673 2.30963 83.89 732.34

46.5518 0.2676 1.95095 1.79 24.99

48.9563 0.1506 1.86061 22.75 178.74

53.7459 0.2342 1.70556 4.34 53.09

58.6036 0.3011 1.57524 10.74 168.78

61.7668 0.2007 1.50195 14.27 149.44

66.4776 0.2007 1.40649 12.33 129.17

68.2783 0.1673 1.37372 10.87 94.87

72.6435 0.3011 1.30156 4.59 72.17

75.2766 0.1632 1.26139 6.83 78.61

The crystallographic structure of copper oxide nanoflowers was investigated by X-ray diffraction analysis (XRD). The typical XRD spectrum [Fig. 18] shows 12 peaks with two major peaks at 2θ = 35.8066 and 38.9983, corresponding to Miller indices (111) and (200). All the peaks are compared with the standard XRD spectrum (AMCSD Card Number: 99-101-0934 of American Mineralogist Crystal Structure Database) [62], and a perfect matching exists between the two spectra. In comparison with standard XRD spectrum, the main peak at 2θ = 35.8066 can be indexed to characteristic diffraction of monoclinic phase of CuO with cell parameters (a=0.4653 nm, b= 0.3410 nm, c= 0.5108 nm). The Miller indices of all the peaks in XRD spectrum are represented by (110), (111), (200), (201), (202), (112), (020), (113), (022), (220), (311) and (222), respectively, corresponding to 12 peaks in Fig. 18.

Energy dispersive X-ray analysis (EDX) of CuO nanoflowers was performed using the FESEM facility of Central Scientific Instruments Organization (CSIO), Chandigarh to determine their chemical composition and stoichiometry. The spectrum (Fig. 19) reveals 3 major peaks: two peaks of copper and one peak of oxygen. Table 2 reveals that the chemical composition of nanoflowers by weight percent and atomic percent are reciprocal. EDX spectrum reveals the chemical purity of CuO nanoflowers as other trace impurities are found to be absent.

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Fig. 19 EDX spectrum of copper oxide nanoflowers prepared by hydrothermal method

Table 2 Quantitative results for copper oxide nanoflowers composition

Element

Line

Weight %

Weight %

Error

Atom %

Atom %

Error

O K 33.60 +/- 0.73 66.77 +/- 1.44

Cu K 66.40 +/- 3.31 33.23 +/- 1.66

Total 100.00 100.00

During the present series of experiments, our aim was to investigate the phenomenon of growth of nanoflowers and exotic patterns instead of copper nanowires. For this purpose, we used both sputter coated and as received anodic alumina templates (AAT) of 100 nm pore diameter. In case of unsputtered, or as received templates, we observed synthesis of polycrystalline copper crystals (Fig. 20a), copper buds acting as nucleation centers for nanowires (Fig. 20b) and copper nanoflowers (Fig. 21) grown on the cathode surface when alumina template was dissolved in 1 M NaOH for 1 hour in a beaker under gentle shaking. These flower shapes have nothing in common with those of figures 12 and 13, grown in polymer templates. Surprisingly, when the experiment was repeated using sputter-coated alumina template of 100 nm pore diameter with a thin film of copper, the growth of nanorods of identical shape was observed in abundance (Fig. 22).

174 Functional Nanomaterials and their Applications

(a) (b)

Fig. 20 (a, b) SEM micrographs showing growth of copper crystals and nanowires

(a) (b)

Fig. 21 (a, b) SEM micrographs of copper nanoflowers grown in AAT of 100 nm pore diameter

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Fig. 22 SEM micrograph of copper nanorods grown in AAT of 100 nm pore diameter

In the case of anodic alumina template (AAT), it was also observed that if the template is not sputter- coated with a conducting film, the copper nanowires were synthesised in the central region and nanoflowers in the peripheral zone [25].

4. Summary

Template synthesis is an elegant method for fabrication of copper nanoflowers, using anodic alumina or polymer templates. Over-deposition plays sterling role in the growth of exotic patterns of copper. It is difficult to determine the exact conditions under which nanoflowers are synthesized along with nanowires. It shows differential deposition of copper on the cathode surface. CuO nanoflowers prepared by CTAB-assisted hydrothermal method, exhibit monoclinic phase and polycrystalline nature. No scientific theory is yet available to explain their exact nature. It has been discovered that nanoflowers have great potential for possible applications in nanotechnology. CuO nanoflowers have been exploited as sensor for hydrogen peroxide (H2O2) [12, 63] and glucose [64],

as well as for field emission properties [65]. Carbon nanotube-wired ZnO nanoflowers have been used as hydrazine sensors [58]. ZnO nanoflowers are finding application in solar cells [66], and ZnO bifunctional nanoflowers loaded with gold nanoparticles show enhanced conversion efficiency in solar cells [67].

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